Improved phosphors could boost young industry

August 13, 2009

CORVALLIS, Ore. - With a billion dollar industry as the potential payoff, Oregon State University researchers are hot on the trail of a better "blue phosphor" that might revolutionize the field of electroluminescence.

Some fundamental advances in the past two years have greatly improved the scientific understanding of what's happening at an atomic and electron transport level in this young field of technology.

Researchers say they're now more optimistic of eventual success - a high quality, full color, electroluminescent "flat panel" display that could be engineered for a variety of uses, from automobile instrument panels to high resolution TV and even exotic types of "virtual reality."

But practical applications of that type may never be possible without a better basic understanding of phosphor electronics that so far has eluded experts around the world, despite millions of dollars of research.

"You can't just type words at random and hope that eventually it will sound like Shakespeare," said John Wager, an OSU professor of electrical and computer engineering. "We hope that our improved understanding of the basic chemistry and physics of electroluminescence will guide us towards the quantum leap in device performance that we are looking for."

Electroluminescence, Wager said, is one of four technologies now used to manufacture flat panel displays that are beginning to replace reliable, but old-fashioned cathode ray tubes for such things as television and computer monitors.

Flat panels are light, convenient and only an inch or two thick. As fundamental problems are solved they could become a major growth industry for the U.S. in general and the high-tech industry of Oregon in particular.

There are relative merits and drawbacks to each of the different technologies being considered for flat panels, but electroluminescence should be able to provide very durable, high definition, reliable devices at low cost - especially in a small format such as instrument displays or miniature TVs.

The drawback for years, Wager said, has been lack of an effective "blue phosphor" that holds the key to full color displays. The lack of full color, low cost and high volume has so far confined electroluminescence to about a $120 million a year industry, a small fraction of its potential.

However, OSU scientists say they just recently have come to understand in electronic detail how one of success stories of electroluminescence - a yellow phosphor based on zinc sulfide doped with manganese - works.

In particular, they've determined that a "static space charge" exists within this yellow phosphor that the researchers did not originally believe was there. This apparently plays a major role in its function as a high quality phosphor for use in electroluminescent devices.

"We now know far better how this yellow phosphor works and performs the way it does," Wager said. "That's important, because it gives us a target to aim at in the engineering of a blue phosphor with similar characteristics."

The pursuit of a good blue phosphor, sort of a Holy Grail for electroluminescence researchers, should now pick up speed, Wager said.

"Some compounds, such as strontium sulfide doped with cerium, are being studied, and we know now that they are not as different as we once thought from the yellow phosphor that works so well," Wager said.

The addition of various impurities to such compounds, so-called "defect engineering," may allow these blue phosphors to possess the "space charge" being sought, he said.

In this quest, the OSU scientists are participating in a "Phosphor Technology Center of Excellence" that includes various other prominent universities and flat-panel display companies. This research is supported at an annual level of about $300,000 by the U.S. Army Research Office and the Defense Advanced Research Projects Agency.

OSU is also collaborating closely with Planar Systems, a Beaverton, Ore., high-tech firm that is a world leader in the manufacture of thin film electroluminescence displays.

"We believe that an improvement by a factor of five to 10 in the brightness of a blue phosphor may be possible," Wager said. "We just have to improve on the materials that nature has given us until we find the right combination."

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